1. Field of the Invention
The present invention relates to a diffractive optical element to be used either at a plurality of wavelengths, or with light in a predetermined band, and a, photographic optical system having the diffractive optical element and, more particularly, to a diffractive optical element suited to be used as a part of a photographic optical system using three or more light beams of different original colors in forming a color image.
2. Description of Related Art
Conventionally, as one of the methods for correcting chromatic aberrations of an optical system, there is known a method of combining two glass materials (lenses) which differ in dispersion from each other.
Unlike this method o: reducing the chromatic aberrations by selectively using two glass materials, it has been known to provide the optical system with a diffractive optical element (hereinafter also called the xe2x80x9cdiffraction gratingxe2x80x9d) as made up in either one of the lens surfaces thereof or somewhere else. Such a method of reducing the chromatic aberrations is disclosed in, for example, xe2x80x9cInternational Lens Design Conferencexe2x80x9d in SPIE Vol. 1354 (1990), Japanese Laid-Open Patent Applications No. Hei 4-21342and No. Hei 6-324262 and U.S. Pat. No. 5,044,706. This method is attained by utilizing such a physical phenomenon that, for a refractive surface and a diffractive surface in an optical system, if their refractive powers are of the same sign, chromatic aberrations for the rays of light of a certain reference wavelength occur in the opposite directions. Further, with such a diffractive optical element, when its periodical structure is changed in pitch as it can be done freely, an effect similar to an aspherical lens is produced. Therefore, the diffractive optical element has an additional great advantage of reducing even mono-chromatic aberrations.
Here, it is in refraction that one ray, even after having refracted, remains one ray. In diffraction, on the other hand, one ray brakes up to a plurality of rays in different orders of diffraction. Therefore, for a case of using the diffractive optical element in an optical system, there is a need to make determination of the grating structure so that a light beam of the useful wavelength region concentrates on a particular order (hereinafter also referred to as the xe2x80x9cdesign orderxe2x80x9d). In a situation when light concentrates on the particular order, the intensities of the diffracted rays in the other orders become low. If the intensity is xe2x80x9c0xe2x80x9d, the corresponding diffracted ray becomes non-existent.
In order to make useful the above-described advantage of the diffractive optical element, it becomes necessary throughout the entire range of predetermined wavelengths including design wavelenghts that the diffraction efficiency for the rays in the design order is sufficiently high. It should also be pointed out that the rays having other orders than the design order focus themselves at different places than the rays of the design order do, becoming flare (light). In an optical system employing the diffractive optical element, therefore, it is of great importance to consider the spectral distribution, too, of diffraction efficiencies of the rays in the design order fully and, further, the behavior of even more rays which are in the orders other than the design order (or the useless diffracted rays).
FIG. 19 shows a diffractive optical element 1 in which a diffraction grating 3 is made up in one layer on a substrate 2. With such a diffractive optical element 1 formed on a surface of an optical system, the rays in particular orders diffract with diffraction efficiencies shown in FIG. 20. The values of the diffraction efficiency are in percentage of the diffracted amount of light at every wavelength to the transmitted amount of light. The reflected light from the grating boundary or the like is not taken into account in the evaluation, because the explanation becomes complicated. In FIG. 20, the abscissa represents the wavelength and the ordinate represents the diffraction efficiency. This diffractive optical element 1 is so designed that the diffraction efficiency in the first order (a solid line curve in FIG. 20) is highest in the predetermined wavelength region. That is, the design order is the first one. Furthermore, the diffraction efficiencies in the orders near to the first one (or (1xc2x11)st orders, namely, zero order and second order) are also depicted for comparison. As shown in FIG. 20, it is in the design order that the diffraction efficiency has the highest value at a certain wavelength (hereinafter referred to as the xe2x80x9cdesign wavelengthxe2x80x9d) and becomes gradually lower toward the ends of the whole spectrum. This decrease of the diffraction efficiency in the design order is translated into an increase of the amount of diffracted rays in the other orders, becoming flare. In addition, in a case where two or more diffraction gratings are used, in particular, the lowering of the diffraction efficiency at the other wavelengths than the design wavelength leads to reduction of the transmittance.
To diminish the lowering of the diffraction efficiency, many previous proposals have been made.
For example, Japanese Laid-Open patent Application No. Hei 9-127322 discloses a diffractive optical element made up in such a way that, as shown in FIG. 21, three different materials of different kinds (for three layers 4, 8 and 5 of diffraction gratings) and two different grating thicknesses d1 and d2 (for the bottom and top gratings 4 and 5) are appropriately selected and that the bottom and top diffraction gratings of an equal pitch distribution are juxtaposed. By this construction and arrangement, a high diffraction efficiency in the design order is realized over the entire visible region, as shown in FIG. 23.
Also, a diffractive optical element capable (if diminishing the lowering of the diffraction efficiency has been proposed in Japanese Laid-Open Patent Application No. Hei 10-133149. As shown in FIG. 22, this diffractive optical element has two layers superimposed one upon another. For the stratification of the layers 4 and 5 in cross-section, the refractive indices and dispersions of their materials and the thicknesses of the gratings in them are made optimum, thus realizing a high diffraction efficiency in the design order over the entire range of visible spectrum.
In another Japanese Laid-Open Patent Application No. Hei 10-104411, with the use of a diffractive optical element of the kinoform type shown in FIG. 19, the grating thickness is adjusted to shift the design wavelength as desired, thus reducing the amount of needless diffracted light in the orders near to the design order.
Of the prior known techniques described above, the one proposed in Japanese Laid-Open patent Application No. Hei 9-127322 has greatly improved the diffraction efficiency in the design order. Therefore, the proportion of the diffracted rays in the orders other than the design order, or the needless diffracted rays, too, is improved. So, the diffractive optical element produces less flare. However, color flare is appreciable in the obtained image. Also, there is no detailed description about the color appearance of flare and the amount of flare.
Meanwhile, Japanese Laid-Open Patent Application No. Hei 10-104411 is concerned with the grating having one diffractive surface like that shown in FIG. 19 (hereinafter called the xe2x80x9cmono-layer DOExe2x80x9d for Diffractive Optical Element). With this regard, it suggests the influence of the color flare due to the light in the needless orders. However, as far as the diffractive optical element in the stratified form of two or more layers (hereinafter called the xe2x80x9cstratified multilayer DOExe2x80x9d) is concerned, nothing is said about the flare.
Using the stratified multilayer DOE described above, the optical system has succeeded in greatly reducing the flare from that when the mono-layer DOE is in use. However, this does not mean that the useless diffracted light is not present at all. So, it is, though little, left to exist. In application to a type of optical system which does not suffer changes of the photo-taking (light-projecting) condition (for example, the reader lens in the copying machine and the projection lens in the liquid crystal projector), the influence of flare is depressed to a negligible level by the stratified multilayer DOE. However, after having conducted many investigations, the inventor of the present invention has found that, for the film camera or video camera, as various photographic conditions are encountered, it sometimes happens that the little remaining of flare gives a serious problem. To show an example, in a case where a light source exists in the scene to be photographed, a correct exposure is usually made not on the light source, but on an object of photographic interest other than the light source. Accordingly, the light source is shot in an over-exposure. For example, assuming that the exposure to the light source is 100 times greater than the correct exposure, then even if the flare is left as little as 2%, because the flare of the light source, too, is 100 times intensified, the flare gets a light amount 2 times as large as the correct exposure. Therefore, it is sure that the flare appears in the picture which will be taken.
As described above, in application of the stratified multilayer DOE to the optical system in the film camera or video camera, the flare becomes problematic with some possibility, no matter however little it may be. In particular, if the flare component has a wavelength dependency, color flare is produced even in the case of the stratified multilayer DOE, being similar to the color light characteristics based on the mono-layer DOE in Japanese Laid-Open Patent Application No. Hei 10-104411.
An object of the present invention is to provide a diffractive optical element having no prominent color flare due to the diffracted rays in the needless orders, and a photographic optical system having the diffractive optical element.
To attain the above object, in accordance with a first aspect of the invention, there is provided a photographic optical system comprising a diffractive optical element having a grating structure in which a plurality of diffraction gratings made from at least two kinds of materials different in dispersion from each other are laminated, and having a plurality of design wavelengths, a maximum optical path length difference in the grating structure being integer times each of the plurality of design wavelengths, the diffractive optical element satisfying the following conditions for each of the design wavelengths xcex0:
0 less than E1(xcex0)+E2(xcex0)+E3(xcex0) less than 0.04xe2x80x83xe2x80x83(1)
0 less than max{E1(xcex0),E2(xcex0),E3(xcex0)}xe2x88x92min{E1(xcex0),E2(xcex0),E3(xcex0)} less than 0.02(2)
where max{E1(xcex0),E2(xcex0),E3(xcex0)} represents a maximum value among E1(xcex0), E2(xcex0) and E3(xcex0), and min{E1(xcex0),E2(xcex0),E3(xcex0)} represents a minimum value among E1(xcex0), E2(xcex0) and E3(xcex0),
where             E1      ⁡              (                  λ          ⁢                      xe2x80x83                    ⁢          0                )              =                                                                      ∫                                                                            D                                              m                        -                        1                                                              ⁡                                          (                                                                        λ                          ⁢                                                      xe2x80x83                                                    ⁢                          0                                                ,                        λ                                            )                                                        ⁢                                      L                    ⁡                                          (                      λ                      )                                                        ⁢                                      F1                    ⁡                                          (                      λ                      )                                                        ⁢                                      T                    ⁡                                          (                      λ                      )                                                        ⁢                                      ⅆ                    λ                                                              +                                                                          ∫                                                                    D                                          m                      +                      1                                                        ⁡                                      (                                                                  λ                        ⁢                                                  xe2x80x83                                                ⁢                        0                                            ,                      λ                                        )                                                  ⁢                                  L                  ⁡                                      (                    λ                    )                                                  ⁢                                  F1                  ⁡                                      (                    λ                    )                                                  ⁢                                  T                  ⁡                                      (                    λ                    )                                                  ⁢                                  ⅆ                  λ                                                                                ∫                                            D              m                        ⁡                          (                                                λ                  ⁢                                      xe2x80x83                                    ⁢                  0                                ,                λ                            )                                ⁢                      L            ⁡                          (              λ              )                                ⁢                      F1            ⁡                          (              λ              )                                ⁢                      T            ⁡                          (              λ              )                                ⁢                      ⅆ            λ                                          E2      ⁡              (                  λ          ⁢                      xe2x80x83                    ⁢          0                )              =                                                                      ∫                                                                            D                                              m                        -                        1                                                              ⁡                                          (                                                                        λ                          ⁢                                                      xe2x80x83                                                    ⁢                          0                                                ,                        λ                                            )                                                        ⁢                                      L                    ⁡                                          (                      λ                      )                                                        ⁢                                      F2                    ⁡                                          (                      λ                      )                                                        ⁢                                      T                    ⁡                                          (                      λ                      )                                                        ⁢                                      ⅆ                    λ                                                              +                                                                          ∫                                                                    D                                          m                      +                      1                                                        ⁡                                      (                                                                  λ                        ⁢                                                  xe2x80x83                                                ⁢                        0                                            ,                      λ                                        )                                                  ⁢                                  L                  ⁡                                      (                    λ                    )                                                  ⁢                                  F2                  ⁡                                      (                    λ                    )                                                  ⁢                                  T                  ⁡                                      (                    λ                    )                                                  ⁢                                  ⅆ                  λ                                                                                ∫                                            D              m                        ⁡                          (                                                λ                  ⁢                                      xe2x80x83                                    ⁢                  0                                ,                λ                            )                                ⁢                      L            ⁡                          (              λ              )                                ⁢                      F2            ⁡                          (              λ              )                                ⁢                      T            ⁡                          (              λ              )                                ⁢                      ⅆ            λ                                          E3      ⁡              (                  λ          ⁢                      xe2x80x83                    ⁢          0                )              =                                                                      ∫                                                                            D                                              m                        -                        1                                                              ⁡                                          (                                                                        λ                          ⁢                                                      xe2x80x83                                                    ⁢                          0                                                ,                        λ                                            )                                                        ⁢                                      L                    ⁡                                          (                      λ                      )                                                        ⁢                                      F3                    ⁡                                          (                      λ                      )                                                        ⁢                                      T                    ⁡                                          (                      λ                      )                                                        ⁢                                      ⅆ                    λ                                                              +                                                                          ∫                                                                    D                                          m                      +                      1                                                        ⁡                                      (                                                                  λ                        ⁢                                                  xe2x80x83                                                ⁢                        0                                            ,                      λ                                        )                                                  ⁢                                  L                  ⁡                                      (                    λ                    )                                                  ⁢                                  F3                  ⁡                                      (                    λ                    )                                                  ⁢                                  T                  ⁡                                      (                    λ                    )                                                  ⁢                                  ⅆ                  λ                                                                                ∫                                            D              m                        ⁡                          (                                                λ                  ⁢                                      xe2x80x83                                    ⁢                  0                                ,                λ                            )                                ⁢                      L            ⁡                          (              λ              )                                ⁢                      F3            ⁡                          (              λ              )                                ⁢                      T            ⁡                          (              λ              )                                ⁢                      ⅆ            λ                                    where                                            ∫                                                            D                  m                                ⁡                                  (                                                            λ                      ⁢                                              xe2x80x83                                            ⁢                      0                                        ,                    λ                                    )                                            ⁢                              L                ⁡                                  (                  λ                  )                                            ⁢                              F1                ⁡                                  (                  λ                  )                                            ⁢                              T                ⁡                                  (                  λ                  )                                            ⁢                              ⅆ                λ                                              =                      xe2x80x83                    ⁢                      ∫                                                            D                  m                                ⁡                                  (                                                            λ                      ⁢                                              xe2x80x83                                            ⁢                      0                                        ,                    λ                                    )                                            ⁢                              L                ⁡                                  (                  λ                  )                                            ⁢                              F2                ⁡                                  (                  λ                  )                                            ⁢                              T                ⁡                                  (                  λ                  )                                            ⁢                              ⅆ                λ                                                                                  =                      xe2x80x83                    ⁢                      ∫                                                            D                  m                                ⁡                                  (                                                            λ                      ⁢                                              xe2x80x83                                            ⁢                      0                                        ,                    λ                                    )                                            ⁢                              L                ⁡                                  (                  λ                  )                                            ⁢                              F3                ⁡                                  (                  λ                  )                                            ⁢                              T                ⁡                                  (                  λ                  )                                            ⁢                              ⅆ                λ                                                        
where
Dmxe2x88x921(xcex0,xcex), Dm(xcexc0,xcex) and Dm+1(xcex0,xcex): diffraction efficiencies for a wavelength xcex in the (mxe2x88x921)st, m-th and (m+1)st orders, respectively, in the diffractive optical element where the m-th order is taken as a design order and the wavelength xcex0 is taken as the design wavelength, L(xcex): a spectral characteristic for the wavelength xcex of a light source,
F1(xcex), F2(xcex), F3(xcex): spectral sensitivity characteristics of light receiving means for detecting light in respective wavelength regions in an image pickup means, where F1(xcex), F2(xcex) and F3(xcex) are arranged in order from the shorter of wavelengths at which spectral sensitivities become maximum, and
T(xcex): a transmittance for the wavelength xcex of the photographic optical system.
Another feature in the first aspect of the invention is that the shortest design wavelength xcex01 among the plurality of design wavelengths of the diffractive optical element satisfies the following condition:
400 nmxe2x89xa6xcex01xe2x89xa6455 nm.
Another feature in the first aspect of the invention is that the longest design wavelength xcex0L among the plurality of design wavelengths of the diffractive optical element satisfies the following condition:
550 nmxe2x89xa6xcexLxe2x89xa6620 nm.
Another feature in the first aspect of the invention is that an interval xcex94xcex0.a between adjacent two of the plurality of design wavelengths of the diffractive optical element satisfies the following condition:
xcex94xcex0.axe2x89xa6220 nm
where xcex94xcex0.a=0.a+1xe2x88x92xcex0.a
1xe2x89xa6axe2x89xa6Lxe2x88x921
where L is the number of the plurality of design wavelengths.
Another feature in the first aspect of the invnetion is that at least one of the plurality of diffraction gratings differs in orientation of grating from the other diffraction gratings.
Another feature in the first aspect of the invention is that a useful wavelength region of the diffractive optical element is a visible spectrum.
Another feature in the first aspect of the invention is that the plurality of diffraction gratings are layered on a substrate and, when the plurality of diffraction gratings are consecutively numbered, from the diffraction grating nearest to the substrate, as the first diffraction grating, the second diffraction grating and up to the i-th diffraction grating, a material from which the first diffraction grating is made is the same as a material of the substrate.
Another feature in the first aspect of the invention is that the plurality of diffraction gratings are arranged in intimate contact or in closely spaced relation.
In accordance with a second aspect of the invention, there is provided a diffractive optical element having a grating structure in which a plurality of diffraction gratings made from at least two kinds of materials different in dispersion from each other are laminated, and having a plurality of design wavelengths, a maximum optical path length difference in the grating structure being integer times each of the plurality of design wavelengths, flare caused by diffracted rays in orders other than the plurality of design wavelengths being made white or a color near to white.
Another feature in the second aspect of the invention is that the shortest design wavelength xcex01 among the plurality of design wavelengths of the diffractive optical element satisfies the following condition:
400 nmxe2x89xa6xcex01xe2x89xa6455 nm.
Another feature in the second aspect of the invention is that the longest design wavelength xcex0L among the plurality of design wavelengths of the diffractive optical element satisfies the following condition:
550 nmxe2x89xa6xcex0Lxe2x89xa6620 nm.
Another feature in the second aspect of the invention is that the plurality of diffraction gratings are arranged in intimate contact or in closely spaced relation.
Another feature in the second aspect of the invention is that a photographic optical system comprises an optical system including the diffractive optical element, and a plurality of light receiving means whose wavelength regions at peak sensitivity are different from each other, and an amount of light of each of diffracted rays in orders other than the plurality of design wavelengths is controlled by the plurality of light receiving means so that the flare becomes white or a color near to white.
The above and further aspects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.